There's a big catch to the HIV prevention pill some are calling '100% effective'

Jeff Chiu/APA little blue pill called Truvada could effectively prevent people from ever getting HIV — but only if they use it perfectly.

A study published this week in Clinical Infectious Diseases followed 657 patients who took Truvada, a prophylactic that protects against HIV, for almost three years. The participants were HIV-negative, but at high risk of contracting the virus. Yet during the course of the study, none of them did.

That's led some publications to hail the drug as "100% effective." It's important to note, however, that this new study was not a clinical trial; there was not a control group that did not take the drug, so it's impossible to say how many more cases there would have been in a non-Truvada group. Still, the results were encouraging.

Back in 2010, the clinical trial that led to the drug's 2012 approval found that Truvada was 99% effective at protecting gay men from the virus. (It had already been approved years earlier as a treatment for people who were already HIV positive.) According to the CDC, it's "been shown to reduce the risk of HIV infection in people who are at high risk by up to 92%."

But there's a catch. In order to achieve these incredible protection rates, you have to take it every day. And humans are not very good at doing that. That's a critical issue if people take the drug imperfectly but proceed as if they're safe. It's "much less effective," the CDC notes, "if it is not taken consistently."

Part of that is because there is serious stigma associated with taking the pill. Opponents of the drug argue that taking it will embolden people to engage in unsafe sexual practices, like forgoing condoms. The phrase "Truvada Whore" has become a pejorative way of referring to people who choose to take the drug to protect themselves from HIV/AIDS.

"The problem," Mark Joseph Stern writes for Slate, "lies in a generational dispute between older gay men, who lived through the worst of the AIDS crisis, and younger ones, who often see HIV as little more than a chronic but manageable disease."

In a great feature story in Out magazine by Tim Murphy, an anonymous doctor explained why he kept the fact that he takes Truvada a secret:

He ... doesn't want to be judged for eschewing condoms from time to time. "Gay men talking about not using condoms is really stigmatized," he says. "Most of us have never known sex without condoms or without threat of a 'deadly disease.' " But he adds passionately, "I think it's a lot to ask an entire generation of gay men to use condoms forever."

Of course, the pill only prevents against HIV, so people taking it can still get other STDs like hepatitis and chlamydia. Condom use and frequent testing is still recommended for people taking Truvada.